Making Money with Adsense?

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What am I missing? I had acyually forgotten that I had adsense on my blog, one day I was checking on old accounts and lo and behold there was money there! Not a lot mind you but there was one day that I had recieved 2.00. So now I am wondering what did I do on those paticular days? Was it my content, new posts or simply luck? If you use adsense what has been your experience? How to optimize earnings?